Sizing Inverter to Inrush Current?

JustPractical · Sep 25, 2021

New to solar, and looking at the MPP "all in one" inverters. Trying to figure out how large I need to go, so I measured inrush and running current on the 4 things I plan to run (a well pump, oil furnace, sump pump and fridge). My question is: do I need to size the inverter to accommodate the worst case of all 4 things starting at the same time? (i.e. do I size to inrush or running?)

For info:
Load / Inrush / Running
Fridge - 1.1A - .87A
Furnace - 28.9A - 7A
Sump pump - 16.88A - 9.4A
Well Pump (240 V) L1 - 23.1A - 6.5A
Well Pump (240v) L2 - 22.5A - 6.6A
(yes, I know the two legs on the well pump should probably be the same, so I chalk that up to the meter, or one of the legs being used for control of pump)

Based on this, total inrush amperage (if everything started at once) is 92.48A which is 11,097 watts (I think)
Total running amps is 30.27A (if everything is on at the same time) which would be 3,632 watts

My concern is the house is normally not inhabited. If everything happened to start at the same time, I don't want to overload the inverter and have it shutdown. If I lived there, I could go down and shed some load and start it back up; but since I'm mostly not there, the inverter might just sit there in the fault state and I would have no heat.

Advice/help/corrections to any mistakes or assumptions above are all appreciated.

My followup question is going to ask about opinions about paralleling two inverters vs just buying the units that are really two units paralleled inside (like the MPP LVX6048). So feel free to opine on that as well if you like.

Thanks in advance!

DerpsyDoodler · Sep 25, 2021

JustPractical said:
New to solar, and looking at the MPP "all in one" inverters. Trying to figure out how large I need to go, so I measured inrush and running current on the 4 things I plan to run (a well pump, oil furnace, sump pump and fridge). My question is: do I need to size the inverter to accommodate the worst case of all 4 things starting at the same time? (i.e. do I size to inrush or running?)

For info:
Load / Inrush / Running
Fridge - 1.1A - .87A
Furnace - 28.9A - 7A
Sump pump - 16.88A - 9.4A
Well Pump (240 V) L1 - 23.1A - 6.5A
Well Pump (240v) L2 - 22.5A - 6.6A
(yes, I know the two legs on the well pump should probably be the same, so I chalk that up to the meter, or one of the legs being used for control of pump)

Based on this, total inrush amperage (if everything started at once) is 92.48A which is 11,097 watts (I think)
Total running amps is 30.27A (if everything is on at the same time) which would be 3,632 watts

My concern is the house is normally not inhabited. If everything happened to start at the same time, I don't want to overload the inverter and have it shutdown. If I lived there, I could go down and shed some load and start it back up; but since I'm mostly not there, the inverter might just sit there in the fault state and I would have no heat.

Advice/help/corrections to any mistakes or assumptions above are all appreciated.

My followup question is going to ask about opinions about paralleling two inverters vs just buying the units that are really two units paralleled inside (like the MPP LVX6048). So feel free to opine on that as well if you like.

Thanks in advance!

Inrush lasts for fractions of a second. What you need to do is decide what you want your system to be able to handle. You know running watts, and you know inrush. Look for a low frequency inverter with surge rating capable of your inrush. generally speaking, low frequency inverters have 3x surge of the listed continuous capacity.

Another option is managed loads. You can take steps to ensure certains loads may not start simultaneously e.g. force a staggered start.

Other options include remote (cloud) management of your inverter and/or certain fault conditions.

I wont comment on the last paragraph. I don't have any knowledge to share. Plenty others will, though.

JustPractical · Sep 27, 2021

DerpsyDoodler said:
Inrush lasts for fractions of a second. What you need to do is decide what you want your system to be able to handle. You know running watts, and you know inrush. Look for a low frequency inverter with surge rating capable of your inrush. generally speaking, low frequency inverters have 3x surge of the listed continuous capacity.

Another option is managed loads. You can take steps to ensure certains loads may not start simultaneously e.g. force a staggered start.

Other options include remote (cloud) management of your inverter and/or certain fault conditions.

I wont comment on the last paragraph. I don't have any knowledge to share. Plenty others will, though.

Thanks for the advice. How can I force a staggered start? I looked into "smart panels" to replace the breaker panel,a nd while that let me control load a bit, there was not much automated about it. I guess I could do a staggered start using relays, but I think that would overcomplicate things. Did you have a different idea?

DerpsyDoodler · Sep 27, 2021

JustPractical said:
Thanks for the advice. How can I force a staggered start? I looked into "smart panels" to replace the breaker panel,a nd while that let me control load a bit, there was not much automated about it. I guess I could do a staggered start using relays, but I think that would overcomplicate things. Did you have a different idea?

A timer and relay is probably simplest. It’s not something I’ve done, had to do, or would choose to do. I would just size the inverter for the load. Ive seen people resort to it because they have what they have and need to make it work.
Other ways to mitigate surge loads are inverter-driven pumps/compressors.

Zil · Sep 28, 2021

You should have a soft-start device on such loads as well pumps.

pollenface · Sep 28, 2021

It's probably a good idea to do some load testing to know what you're inverter can handle all at once. Even splitting the load between 2 inverters might be an idea if you like redundancy (with double the standby draw).

I learned yesterday afternoon that when the PV array is not in full sun I can't run my pressure cleaner for more than a few minutes without tripping the breaker.

MichaelK · Sep 28, 2021

The best advice I would give you is to forget trying to implement any plan utilizing multiple MPP units but get a serious low-frequency inverter instead. I'm using a Schneider XW6848, and my inrush starting loads are significantly higher then yours, in the 36-38A range.

I would never tinker around with a soft-starter for something like a well-pump. Having pulled a dead pump out of the ground, I know what to experiment with and what not to. Feed your well-pump only what it was designed to receive.

JustPractical · Sep 29, 2021

MichaelK said:
The best advice I would give you is to forget trying to implement any plan utilizing multiple MPP units but get a serious low-frequency inverter instead. I'm using a Schneider XW6848, and my inrush starting loads are significantly higher then yours, in the 36-38A range.

I would never tinker around with a soft-starter for something like a well-pump. Having pulled a dead pump out of the ground, I know what to experiment with and what not to. Feed your well-pump only what it was designed to receive.

I agree with not touching the well pump. On large motors the soft start keeps them from flying off their mounts, but my well pump is hundreds of feet down - i'll let sleeping dogs lie.

As for going with a seperate inverter instead - I'm new to all this and liked the idea of having one chunk of hardware that could take AC in to charge the batter ies as well as feed a load, and when AC failed it could switch to feeding load with battery power, and charging off the solar panel side. I'm assuming there are some "smart" things I can put before the inverter - but can they do all the fancy "switching" depending on if Utility AC is available or not?

MichaelK · Sep 29, 2021

JustPractical said:
As for going with a seperate inverter instead - I'm new to all this and liked the idea of having one chunk of hardware that could take AC in to charge the batter ies as well as feed a load, and when AC failed it could switch to feeding load with battery power, and charging off the solar panel side. I'm assuming there are some "smart" things I can put before the inverter - but can they do all the fancy "switching" depending on if Utility AC is available or not?

I think what you are referring to is what's called a "hybrid inverter", which the Schneider XW, and Conext series are. Also the Outback Radian series.
They have three sets of AC terminals. ACout, ACin#1, and ACin#2. ACout is powered by your battery, and the inverter converts your lower voltage DC in 120/240VAC. ACin#1 is the grid connection. The inverter accepts power from the grid, and charges the batteries with that. ACin#2 is for your generator.

I wired a 4-prong split-phase 240V socket/extension that I plug into the generator socket on one end, and a generator input socket in the wall connected to ACin#2 at the other end. With the generator running, the inverter accepts the generator's AC and converts it into battery charging DC.

You can go in to the inverter's settings to optimize either grid output, or grid charging, depending on what your solar input is. I've never fiddled with this though because I'm totally off-grid.

JustPractical · Sep 29, 2021

How is the Schneider XW, Conext series, and Outback Radian different than the Sol-Ark, MPP and Growatt? They all seem to be "all in one" solutions. Huge price differneces, so I assume I'm missing a lot. (and if it makes a difference, I have zero plans of tying back to the grid - just looking to give parts of the house backup power)

DerpsyDoodler · Sep 29, 2021

JustPractical said:
How is the Schneider XW, Conext series, and Outback Radian different than the Sol-Ark, MPP and Growatt? They all seem to be "all in one" solutions. Huge price differneces, so I assume I'm missing a lot. (and if it makes a difference, I have zero plans of tying back to the grid - just looking to give parts of the house backup power)

It sounds like you want an inverter that synchronizes waveform. they usually also have built-in transfer switches. Inverters like this are capable of seemlessly switching from grid to battery and back, without interrupting loads. This is similar to the way UPS' work. If you don't need seemless continuity, a simple low frequency inverter and an external transfer switch. the transfer switch could be either manual or auto.

My inverter, the samlex evo 4024, is a low frequency 4kw inverter. it has a built-in 2 way auto-transfer switch, it synchronizes wave form, and has a deep surge capability. I spent less than $1500 USD on this unit, including the inverter remote.

mrzed001 · Sep 30, 2021

JustPractical said:
New to solar, and looking at the MPP "all in one" inverters. Trying to figure out how large I need to go, so I measured inrush and running current on the 4 things I plan to run (a well pump, oil furnace, sump pump and fridge). My question is: do I need to size the inverter to accommodate the worst case of all 4 things starting at the same time? (i.e. do I size to inrush or running?)

For info:
Load / Inrush / Running
Fridge - 1.1A - .87A
Furnace - 28.9A - 7A
Sump pump - 16.88A - 9.4A
Well Pump (240 V) L1 - 23.1A - 6.5A
Well Pump (240v) L2 - 22.5A - 6.6A
(yes, I know the two legs on the well pump should probably be the same, so I chalk that up to the meter, or one of the legs being used for control of pump)

Based on this, total inrush amperage (if everything started at once) is 92.48A which is 11,097 watts (I think)
Total running amps is 30.27A (if everything is on at the same time) which would be 3,632 watts

My concern is the house is normally not inhabited. If everything happened to start at the same time, I don't want to overload the inverter and have it shutdown. If I lived there, I could go down and shed some load and start it back up; but since I'm mostly not there, the inverter might just sit there in the fault state and I would have no heat.

Advice/help/corrections to any mistakes or assumptions above are all appreciated.

My followup question is going to ask about opinions about paralleling two inverters vs just buying the units that are really two units paralleled inside (like the MPP LVX6048). So feel free to opine on that as well if you like.

Thanks in advance!

Yes, you have to scale the system for inrush.
If you can limit the equipment count that stats on the exactly same time, that is the best solution (like with cheap Sonoff relays).
If not then you need a big inverter.

HF inverters "say" they can handle x2 inrush ... do not count on that. I would say max x1,2-1,4 of its max power.
LF inverters can handle x2-x3 inrush ... but costs x2 and large and heavy.

Other solution is an EU 240V inverter + autotransformer

Also if the inverter stops because of an overload ... they have a setting and they start up again after some seconds.

JustPractical · Sep 30, 2021

DerpsyDoodler said:
It sounds like you want an inverter that synchronizes waveform. they usually also have built-in transfer switches. Inverters like this are capable of seemlessly switching from grid to battery and back, without interrupting loads. This is similar to the way UPS' work. If you don't need seemless continuity, a simple low frequency inverter and an external transfer switch. the transfer switch could be either manual or auto.

My inverter, the samlex evo 4024, is a low frequency 4kw inverter. it has a built-in 2 way auto-transfer switch, it synchronizes wave form, and has a deep surge capability. I spent less than $1500 USD on this unit, including the inverter remote.

I believe I do need to sync waveforms. I need to feed a well pump with split phase 240 volts (120 volt x2 with each wave 180 degrees off from the other). It doesn't need to be instantaneous, just needs to go from grid to battery automatically when the grid goes down. It's an unoccupied cabin, so when the power goes down, I need something that can automatically switch to battery, and then attempt to charge the battery from solar (until the power comes back up, then automatically switch to charging the battery from grid).

JustPractical · Sep 30, 2021

mrzed001 said:
Yes, you have to scale the system for inrush.
If you can limit the equipment count that stats on the exactly same time, that is the best solution (like with cheap Sonoff relays).
If not then you need a big inverter.

HF inverters "say" they can handle x2 inrush ... do not count on that. I would say max x1,2-1,4 of its max power.
LF inverters can handle x2-x3 inrush ... but costs x2 and large and heavy.

Other solution is an EU 240V inverter + autotransformer

Also if the inverter stops because of an overload ... they have a setting and they start up again after some seconds.

I was looking at the Growatt, but people seemed to have less problems with the MPP (saw Will using them, and figured "take the easy route"). I'm assuming these are both high freq. inverters, and thus the lower "max capacity" for starting/inrush current. The Outbacks, Schneiders, and Victron (and even the sol-Arks) seem to be over 2x the price (and in comparing, I'm using higher overall capacities for AC out to account for the lower ability to handle starting currents.

If I got something like the samlex evo 4024, I *think* I need to run two of them in parallel to get the 240 volt split phase. Also, I think I need to get a solar charge controller. I'm assuming I would run an array to each inverter.

mrzed001 · Sep 30, 2021

JustPractical said:
I was looking at the Growatt, but people seemed to have less problems with the MPP (saw Will using them, and figured "take the easy route"). I'm assuming these are both high freq. inverters, and thus the lower "max capacity" for starting/inrush current.

Yes, both are HF inverters. MPP Solar makes this inverters for a decade. Growatt is relatively new in this part.

JustPractical said:
The Outbacks, Schneiders, and Victron (and even the sol-Arks) seem to be over 2x the price (and in comparing, I'm using higher overall capacities for AC out to account for the lower ability to handle starting currents.

At least 2x price. Like for Victron you have to buy 1-2 separate MPPT and that puts extra on the price (like 3x at the end)
Schneider, Victron, Sol-ark inverters are very good products if you have the extra money for them.

One thing I always suggest to consider is the redundancy.
If the single Victron, Sol-ark inverter dies (and it happens sometime, they are not failure proof too) then you sit in the dark until they repair or replace it (or you buy a new expensive one after warranty ... or because it is in the repair shop since a week and you need power)
But if you have 2-3 cheap inverters then they are not used as heavily as a single inverter (load share).
And if one of them dies the other(s) can run your house without any restriction (or some restrictions).
Also cheap (a mobile phone cost more nowadays).

DerpsyDoodler · Sep 30, 2021

JustPractical said:
I believe I do need to sync waveforms. I need to feed a well pump with split phase 240 volts (120 volt x2 with each wave 180 degrees off from the other). It doesn't need to be instantaneous, just needs to go from grid to battery automatically when the grid goes down. It's an unoccupied cabin, so when the power goes down, I need something that can automatically switch to battery, and then attempt to charge the battery from solar (until the power comes back up, then automatically switch to charging the battery from grid).

An evo 4024 does not do split phase. I've heard Schneider makes good split phase output inverters. Victron also has a line of inverters that do split phase and have several options to choose from. Based on a high level understanding of what you want to do, I suggest you start looking at the victron quattro line, and branch out to different models or other brands from there. Victron is the premium standard in this space. As a starting point, shopping victron would be a good educational tool to know whats possible.

Shunt Czar · Sep 30, 2021

Your other surges are so big that it probably doesn't matter, but the fridge is closer to 10 amps:

See at 8:30:

JustPractical · Oct 1, 2021

Shunt Czar said:
Your other surges are so big that it probably doesn't matter, but the fridge is closer to 10 amps:

See at 8:30:

Interesting. I measured 1.1 on the inrush and .87 on the run. I *think* I measured it properly, but who knows for sure...

JustPractical · Oct 1, 2021

Found this in another thread - can use on well pump feed to keep sump pump from running at the same time.

Amazon.com: AC Current Sensing Switch Adjustable Current 0.2~30A Flameproof ABS Self-Powered Sensing Switch Users Manual(Normally Open): Industrial & Scientific

www.amazon.com

Shunt Czar · Oct 1, 2021

JustPractical said:
Interesting. I measured 1.1 on the inrush and .87 on the run. I *think* I measured it properly, but who knows for sure...

Note that in the video it was measured with an oscilloscope. All these little power meters may not catch the whole event, or are averaging it.

Sizing Inverter to Inrush Current?

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